A modern communication system is expected to provide reliable data transmission for a variety of applications, such as voice and data applications. In a point-to-multipoint communications context, known communication systems are based on frequency division multiple access (FDMA), time division multiple access (TDMA), code division multiple access (CDMA), and perhaps other multiple access communication schemes.
A CDMA system may be designed to support one or more CDMA standards, such as (1) the “TIA/EIA-95 Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System” (this standard with its enhanced revisions A and B may be referred to as the “IS-95 standard”), (2) the “TIA/EIA-98-C Recommended Minimum Standard for Dual-Mode Wideband Spread Spectrum Cellular Mobile Station” (the “IS-98 standard”), (3) the standard sponsored by a consortium named “3rd Generation Partnership Project” (3GPP) and embodied in a set of documents known as the “W-CDMA standard,” (4) the standard sponsored by a consortium named “3rd Generation Partnership Project 2” (3GPP2) and embodied in a set of documents including “TR-45.5 Physical Layer Standard for cdma2000 Spread Spectrum Systems,” the “C.S0005-A Upper Layer (Layer 3) Signaling Standard for cdma2000 Spread Spectrum Systems,” and the “TIA/EIA/IS-856 cdma2000 High Rate Packet Data Air Interface Specification” (the “cdma2000 standard” collectively), (5) the 1xEV-DO standard, and (6) certain other standards.
A wireless access terminal, for example, a terminal in a wireless CDMA system, may receive data transmissions from one or more base stations on forward link or links. The signal transmitted by a particular base station may reach the terminal through multiple propagation paths. The received signal at the terminal may include one or more signal instances (also known as multipath components) of the signal transmitted by the base station. The word “multipath” refers to the existence of multiple propagation paths along which a signal travels from a transmitter (e.g., a base station) to a receiver (e.g., an access terminal). Each of the multipath components is also subjected to the varying physical channel response, noise, and interference. The terminal may employ an equalizer to compensate for the channel response and the multipath distortion. An equalizer may be an equalization filter with a number of delay elements and multiplication coefficients at taps corresponding to the delay elements. Some equalization techniques and equalizers are described in a commonly-assigned U.S. Pat. No. 7,301,990, entitled Equalization of Multiple Signals Received for Soft Handoff in Wireless Communication Systems; and in a commonly assigned U.S. Pat. No. 6,522,683, entitled Method and Apparatus for Adaptive Linear Equalization for Walsh Covered Modulation.
Pilot signals may be used for estimating the physical channel between a transmitter and a receiver, for example, from the base station to the access terminal in the CDMA system. A pilot signal is a signal carrying a predefined data sequence, so that distortion of the pilot can be attributed to the transmission channel, and the transmission channel can thus be estimated from the received pilot.
The pilot may be transmitted at well defined, periodic intervals of the forward link. In some CDMA systems, for example, a forward link is defined in terms of frames. A frame may include sixteen time slots. Each time slot may be 2048 chips long, corresponding to a 1.67 millisecond slot duration, and, consequently, a frame with 26.67 millisecond duration. Each slot may be divided into two half-slots, with a pilot burst of 96 chips transmitted in the middle of each half-slot. The remainder of each half-slot is occupied by two traffic carrying portions of about 400 chips each, and media access control (MAC) portions.
With each pilot burst, the equalizer is trained and its tap coefficients are adapted based on the estimate obtained with the pilot burst. The coefficients thus obtained are then used to demodulate the traffic portions on each side of the pilot burst. Because the coefficients obtained from training on a particular pilot burst are used to demodulate traffic following the pilot burst in time, the method is anti-causal.
In fast changing channel conditions, the channel may undergo a substantial variation between the time of the pilot burst and the actual transmission and receipt of data, particularly for the data that is most distant in time from the pilot burst. Proper equalizer training is important for equalizer performance and, consequently, for receiver performance. Therefore, there is a need in the art for apparatus, methods, and articles of manufacture that improve matching of equalizer coefficients to the actual transmission channel conditions at the time of the data transmission and receipt. There is also a need in the art for receivers with such improved equalizers. There is a further need in the art for wireless communication systems that employ such receivers.
Signal to noise and interference ratio (SINR) for the signal is measured during pilot bursts as well, and then used for scaling equalizer output before feeding it to a block that calculates log likelihood ratio, and/or for other processing of the transmitted information. Consequently, obtaining good SINR estimates is also important for receiver performance. Therefore, there is a need in the art for apparatus, methods, and articles of manufacture that improve SINR estimates of the actual transmission channel conditions at the time of the data transmission and receipt. There is also a need in the art for receivers that use such improved SINR estimates. There is a further need in the art for wireless communication systems that employ such receivers.